I've figured out how to fix this error.
Apparently a very simple fix (for me and hopefully you too).
All you need to do is go to:
Find your app you created, click Edit Settings, and make sure that you have App on Facebook checked. Then enter your domain and a link to https:// (even if you don't have one, just enter it in).
Then my plugin started working.
Now, I do not remember the instructions telling you to do this. In fact it does not. If you see "nux_app_settings.png" and it does not show or say to do this:
Step 2: Set up the App
Next, set up your app so that it looks like the settings below. Make sure you set your app's icon and image, too. If you already have an app and skipped Step 1, you can view your app settings by going to https://developers.facebook.com/apps.
Here are for some recommendations for filling this form out, based on where this plugin is installed.
Once I do the plugin works. Also its worth mentioning to get the developer version from their GitHub, since it fixes some other problems (https://github.com/facebook/wordpress/).
If you are still having issues, let me know. I'll try to recreate my steps for you.